Characterization of the growth promotion and defense enhancement of soybean plants driven by seed treatment of multiple soybean-associated beneficial bacteria

We screened soybean-associated bacterial strains from soybean fields in Louisiana, USA, based on various biological activities beneficial for soybean growth and health. Furthermore, we constructed sets of synthetic bacterial community (SBC) containing multiple strains of soybean-associated beneficial bacteria (SABB) having different types of beneficial activities and tested their effects of seed treatment on soybean growth and disease resistance. We found that all three sets of SBC (i.e., Set-1, Set-2, and Set-3) tested promoted soybean growth and yield significantly through seed treatment, showing better performance than the most effective SABB strain Pseudomonas putida SABB7 alone and the commercial seed-treating product included for comparison. Our analysis of soybean microbiomes in the root endosphere and rhizosphere based on 16S rDNA sequence profiles revealed that Bradyrhizobium elkanii, a symbiotic bacterium of soybean, was enriched in both compartments by seed treatment with Set-2 or Set-m4, which were the best-performing bacterial mixture among the three SABB sets and the most effective subset of Set-2, respectively. In addition, the soybean gene expression profile determined by RNA-seq revealed that seed treatment with Set-2 or Set-m4 made soybean plants grown from the treated-seeds induce a higher level of defense-related genes upon infection by the fungal pathogen Rhizoctonia solani compared to those from untreated seeds. These experimental results strongly suggest that the beneficial effects of the bacterial mixtures on plant growth and defense through seed treatment are largely mediated by the change of soybean-associated microbiomes that enriches beneficial components such as B. elkanii and the defense-priming effect that induces robust defense responses upon pathogen infection. This study provides a valuable insight into the development of innovative and sustainable management strategies through seed treatment of beneficial microbes in a form of SBC for soybean and further other major crops. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/575074v1_ufig1.gif" ALT="Figure 1"> View larger version (56K): org.highwire.dtl.DTLVardef@41f817org.highwire.dtl.DTLVardef@106c4b4org.highwire.dtl.DTLVardef@15c1555org.highwire.dtl.DTLVardef@7cbba4_HPS_FORMAT_FIGEXP M_FIG C_FIG